The work was primarily funded by the National Science Foundation (NSF), Chemistry Division (award number CHE-1507370) is acknowledged. Ayman M. Karim and Wenhui Li acknowledge partial financial support by 3M Non-Tenured Faculty Award. This research used resources of the Advanced Photon Source (beamline 12-ID-C, user proposal GUP-45774), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors would like to thank Yubing Lu, a Ph.D. candidate in the Chemical Engineering Department at Virginia Tech for his kind help with the SAXS measurements. The presented work was partly executed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396.

There is no conflict of interest to report.

In this study, we presented a powerful methodology to examine the effect of capping ligands on the nucleation and growth of metal nanoparticles. We synthesized Pd nanoparticles in different solvents (toluene and pyridine) using Pd acetate as the metal precursor and TOP as the ligand.We used in situ SAXS to extract the concentration of reduced atoms (nucleation and growth events) as well as the concentration of nanoparticles (nucleation event), where both experimental observables were used as the model inputs. In addition, by considering the slope of the concentration of the nanoparticles and concentration of the atoms at the early reaction time, our methodology (the use of in situ SAXS and kinetic modeling), allowed us to estimate the upper and lower bonds for the nucleation and growth rate constants (more details can be found in ref. 14, which was the first study to decouple the contributions of nucleation and growth to the total metal reduction).
There are three critical steps in systematically examining the effects of ligand-metal binding on the nucleation and growth of colloidal nanoparticles: (i) measuring the evolution of size as well as the concentration of nanoparticles (steps 4.1-4.3). This is an important step as it can provide more detailed information on both the nucleation and growth events, (ii) developing a robust kinetic model, which explicitly accounts for the reactions of capping ligands with the metal and also includes the most relevant reactions during the formation and growth of nanoparticles (step 6.4), and (iii) constructing an appropriate link between the experimental observables and those extracted from the model (e.g., size measured experimentally versus size extracted from the model).
It is important to note that due to the small size of the particles (&#60; 10 nm in diameter), and the fast nucleation and growth rates in the beginning of the reaction, a high energy and high flux X-ray beam is needed for obtaining in situ data, which can be only realized at the synchrotron. Even with synchrotron beams, it is difficult to capture any size below 0.5 nm unless the concentration of the particle is high enough. A rule of thumb principle is that SAXS intensity reduces with 6th power of the particle size but it is only linearly proportional to the concentration of the nanoparticles. In addition, for smaller nanoparticles, data acquisition up to much higher wave vector q (wider angle) is required, where the background scattering from solvents become more significantly detrimental to signal to noise ratio. This limits the size and concentration of small nanoparticles that can be detected in the early stages of the reaction, especially when the nucleation is slow and continuous as shown in this work. However, while the high energy/flux allows the acquisition of in situ data, the beam can also cause damage to the sample (agglomeration of nanoparticles and/or deposition on the cell walls). Therefore, in step 5.1, the beam energy and X-ray exposure time need to be tested and adjusted to the level that provides the best data quality (signal to noise ratio) for the detection of small nanoparticles in the early stages of the reaction without causing damage to the sample. The troubleshooting has to be done at the synchrotron during the in situ SAXS measurement, i.e., to monitor the SAXS spectra and ensure that no agglomeration/precipitation occurs during the synthesis. Through a few tests, the beam energy was finally set at 18 keV with an appropriate exposure time (0.1 s) to capture enough signal, and hence, the small Pd nanoparticle size in the early stage of reaction. We also note that while the current kinetic model does not account for agglomeration, if such growth mechanism is dominant, the model can be modified to include agglomeration steps (for example, B + B &#8594; C and B + C &#8594; 1.5C, where B and C represent the small and larger nanoparticles, respectively)1. However, agglomeration as well as other modes of growth (i.e., Ostwald and digestive ripening)40 would be best described by population based models24,25,32,33.
As already discussed in the manuscript, the underlying mechanism governing the nanoparticle nucleation and growth is poorly understood, particularly in the presence of coordinating ligands. For example, recent studies showed that TOP-Pd binding lowers the nucleation and growth rate of Pd nanoparticles14,15,16,30. Therefore, we accounted explicitly for the ligand-metal binding in our kinetic model. What distinguishes our method from other relevant studies is that our ligand-based model considers the ligand binding with both the precursor and surface of metal nanoparticle as reversible reactions and no priori assumptions are made on whether the ligands are in equilibrium with either of them. In addition, unlike previous studies where only one experimental observable (either size33 or concentration of atoms23, etc.) was used for model verification, our ligand-based model uses both the particle size and concentration of nanoparticles as model inputs. Therefore, it allows us to obtain more accurate estimates for the reaction rate and equilibrium constants.
Using our proposed methodology, we demonstrated the predictive power of our ligand-based model. In this regard, we showed that the model can predict the synthesis conditions to obtain nanoparticles with various sizes, which as a result minimizes the need for trial and error. Furthermore, with this simple &#34;heat-up&#34; synthesis method, the nanoparticle size can be tuned by changing the type of solvent or the metal concentration. These different sized Pd nanoparticles can have potential applications in catalysis, drug delivery, and sensors15,41. The presented synthesis strategy along with the kinetic modeling can be potentially used to provide insights on the role of capping ligands in the nucleation and growth of different types of nanoparticles to guide their controlled synthesis.
For future work, we direct our research toward developing kinetic models with the ability of predicting the size distribution during the synthesis. In addition, we will further investigate the validity of our ligand-based model under different experimental conditions, including different temperature ranges and different types of ligands and metals.

Controlled synthesis of metallic nanoparticles is of great importance due to the large applications of nanostructured materials in catalysis, photovoltaic, photonics, sensors, and drug delivery1,2,3,4,5. To synthesize the nanoparticles with specific sizes and size distribution, it is vital to understand the underlying mechanism for the particle nucleation and growth. Nevertheless, obtaining nanoparticles with such criteria has challenged the nano-synthesis community due to the slow progress in understanding the synthesis mechanisms and the lack of robust kinetic models available in the literature. In 1950s, LaMer proposed a model for the nucleation and growth of sulfur sols, where there is a burst of nucleation followed by a diffusion-controlled growth of nuclei6,7. In this proposed model, it is postulated that the monomer concentration increases (due to the reduction or decomposition of the precursor) and once the level is above the critical supersaturation, the energy barrier for particle nucleation can be overcome, resulting in a burst nucleation (homogeneous nucleation). Owing to the proposed burst nucleation, the monomer concentration drops and when it falls below the critical supersaturation level, the nucleation stops. Next, the formed nuclei are postulated to grow via the diffusion of monomers towards the nanoparticles surface, while no additional nucleation events occur. This results in effectively separating the nucleation and growth in time and controlling the size distribution during the growth process8. This model was used to describe the formation of different nanoparticles including Ag9, Au10, CdSe11, and Fe3O412. However, several studies illustrated that the classical nucleation theory (CNT) cannot describe the formation of colloidal nanoparticles, in particular for metallic nanoparticles where the overlap of the nucleation and growth is observed1,13,14,15,16,17. In one of those studies, Watzky and Finke established a two-step mechanism for the formation of iridium nanoparticles13, in which a slow continuous nucleation overlaps with a fast nanoparticle surface growth (where growth is autocatalytic). The slow nucleation and fast autocatalytic growth were also observed for different types of metal nanoparticles, such as Pd14,15,18, Pt19,20, and Rh21,22. Despite recent advances in developing nucleation and growth models1,23,24,25, the role of the ligands is often ignored in the proposed models. Nevertheless, ligands are shown to affect the nanoparticles size14,15,26 and morphology19,27 as well as the catalytic activity and selectivity28,29. For example, Yang et al.30 controlled the Pd nanoparticle size ranging from 9.5 and 15 nm by varying the concentration of trioctylphosphine (TOP). In the synthesis of magnetic nanoparticles (Fe3O4), the size noticeably decreased from 11 to 5 nm when the ligand (octadecylamine) to metal precursor ratio increased from 1 to 60. Interestingly, the size of Pt nanoparticles was shown to be sensitive to the chain length of amine ligands (e.g., n-hexylamine and octadecylamine), where smaller nanoparticle size could be obtained using longer chain (i.e., octadecylamine)31.
The size alteration caused by different concentration and different types of the ligands is a clear evidence for the contribution of ligands in the nucleation and growth kinetics. Unfortunately, few studies accounted for the role of ligands, and in these studies, several assumptions were often made for the sake of simplicity, which in turn make these models applicable only for specific conditions32,33. More specifically, Rempel and co-workers developed a kinetic model to describe the formation of quantum dots (CdSe) in the presence of capping ligands. However, in their study, the binding of the ligand with nanoparticle surface is assumed to be at equilibrium at any given time32. This assumption might hold true when the ligands are in large excess. Our group recently developed a new ligand-based model14 which accounted for the binding of capping ligands with both the precursor (metal complex) and the surface of nanoparticle as reversible reactions14. In addition, our ligand-based model could potentially be used in other metal nanoparticle systems, where the synthesis kinetics seem to be affected by the presence of the ligands.
In the current study, we use our newly developed ligand-based model to predict the formation and growth of Pd nanoparticles in different solvents including toluene and pyridine. For our model input, in situ SAXS was utilized to obtain the concentration of nanoparticles and size distribution during the synthesis. Measuring both the size and concentration of particles, complemented by kinetic modeling, allows us to extract more precise information on the nucleation and growth rates. We further demonstrate that our ligand-based model, which explicitly accounts for the ligand-metal binding, is highly predictive and can be used to design the synthesis procedures to obtain nanoparticles with desired sizes.

<table border="0" cellpadding="0" cellspacing="0" width="778">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">palladium acetate (Pd(OAc)2)</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">520764</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">anhydrous acetic acid</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">338826</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">trioctylphosphine</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">718165</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">pyridine</td>
			<td style="width:203px;">MilliporeSigma</td>
			<td style="width:119px;">PX2012-7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">toluene</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">244511</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">1-hexanol</td>
			<td style="width:203px;">SIAL</td>
			<td style="width:119px;">471402</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">N8 Horizon SAXS</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;">A32-X1</td>
			<td></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:291px;">glovebox</td>
			<td style="width:203px;">Vaccum Atmospheres Co.</td>
			<td style="width:119px;">109035</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">MR HEI-TEC 115V Hotplate</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5053000000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">hotplate Monoblock insert</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5058000800</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">heat-On 25-ml insert</td>
			<td style="width:203px;">Heidolph</td>
			<td style="width:119px;">5058006200</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">7 mL vials</td>
			<td style="width:203px;">SUPELCO</td>
			<td style="width:119px;">27518</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">micro stir bar PTFE&#160;</td>
			<td style="width:203px;">VWR</td>
			<td style="width:119px;">58948-353</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">egg-Shaped Bars&#160;</td>
			<td style="width:203px;">Fisherbrand&#8482;&#160;</td>
			<td style="width:119px;">14-512-121</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">25 mL round bottom flasks</td>
			<td style="width:203px;">ALDRICH</td>
			<td style="width:119px;">Z167495</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">quartz capillary</td>
			<td style="width:203px;">Hampton Research</td>
			<td style="width:119px;">HR6-148</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">MATLAB R2016b</td>
			<td style="width:203px;">MathWorks</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">Bruker SAXS 1.0v</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:291px;">Diffrac Measurement Center 4.0v</td>
			<td style="width:203px;">Bruker</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

ligand-mediated nucleation and growth of palladium metal nanoparticles

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

To systematically examine whether the capping ligands alter the kinetics of nucleation and growth, we took the two following approaches: (i) the binding of the ligand with the metal was not considered in the kinetic model similar to previous studies (i.e., the nucleation and autocatalytic growth) (ii) the reversible binding of capping ligand with the precursor and surface of the nanoparticle was taken into account in the model (i.e., ligand-based model described in Protocol). Regarding the Pd synthesis in toluene, as shown in Figure 1, without accounting for the ligand-metal binding, the model failed to capture the time evolution of the nanoparticles concentration (<img alt="Equation 72" src="/files/ftp_upload/57667/57667eq72v3.jpg" />) and concentration of Pd atoms (<img alt="Equation 73" src="/files/ftp_upload/57667/57667eq73v3.jpg" />). As an alternative, we implemented our newly developed kinetic model (Figure 2) and as depicted in Figure 3, the model accurately predicts our in situ data (both <img alt="Equation 72" src="/files/ftp_upload/57667/57667eq72v3.jpg" /> and <img alt="Equation 73" src="/files/ftp_upload/57667/57667eq73v3.jpg" /> during reaction).This further indicates that the capping ligands indeed affect the nucleation and growth kinetics of Pd nanoparticles.
Estimating the rate constants (Table 1) from the model further enables us to obtain useful information on the kinetics of the nanoparticle formation. In this regard, Figure 4A shows the comparison between the nucleation and growth rates (as estimated from the model) and the results clearly reveal that nucleation is slow while the growth is fast, which agrees well with previous studies1,14. Both modeling and experimental results demonstrate that the metal precursor/monomer does not undergo burst nucleation. This is illustrated by the in situ SAXS and modeling results where the nucleation continues till the end of synthesis (Figure 3B and Figure 4A). The continuous formation of nuclei, therefore, contradicts the LaMer burst nucleation and growth model but supports the continuous nucleation reaction in the Finke-Watzky two step mechanism. In addition, the nucleation can be fitted by pseudo-first order; however, we cannot exclude the possibility that the nucleation could be higher in order. Herein, as shown in Figure 4B, the ligand plays a central role in the continuity of nucleation by further binding to the nanoparticle surface and reducing the concentration of active sites (i.e., [B]). This drastically decreases the particle growth rate and expands the time window for the nucleation throughout the synthesis. In addition, our current results presented in this work in combination with our previous study14 (where the synthesis was conducted under different experimental conditions) indicate that the ligand and precursor concentrations do not have a significant effect on the rate and equilibrium constants, which shows the chemical fidelity between the model and the real system.
Next, we probed the applicability of our ligand-based model to a different solvent system, where pyridine was used as a solvent instead of toluene. We can see that despite the significant difference observed for the nucleation and growth kinetics in pyridine compared to toluene (Figure 5 and Table 1), the model accurately captures the in situ data, <img alt="Equation 72" src="/files/ftp_upload/57667/57667eq72v3.jpg" /> and <img alt="Equation 73" src="/files/ftp_upload/57667/57667eq73v3.jpg" />, and allows for more accurate estimation of rate constants (Table 1). One of the important features that makes a kinetic model robust is that it should be able to predict synthetic conditions for achieving nanoparticles with desired sizes. Therefore, we implemented our ligand-based model (using the same rate constants reported in Table 1) to predict the size under different concentrations of metal precursor, Pd(OAc)2, in pyridine. Figure 6 shows that the model can provide a very accurate estimation of the nanoparticle size under different concentrations of the metal precursor. The modeling as well as the experimental results demonstrate that the nanoparticles become larger in size at higher precursor concentration. This is because the growth is second order kinetics while the nucleation is first order which makes the growth faster at higher precursor concentration14.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57667/57667fig1.jpg" /> 
Figure 1. Experimental and two-step modeling results for the synthesis of Pd nanoparticles in toluene: (A) concentration of Pd atoms and (B) concentration of nanoparticles. The rate constants are <img alt="Equation 36" src="/files/ftp_upload/57667/57667eq36v3.jpg" />= <img alt="Equation 74" src="/files/ftp_upload/57667/57667eq74v3.jpg" />s-1 and&#160;<img alt="Equation 75" src="/files/ftp_upload/57667/57667eq75v3.jpg" />= <img alt="Equation 76" src="/files/ftp_upload/57667/57667eq76v3.jpg" /> L.mol-1.s-1. Experimental condition: [Pd(OAc)2]=25 mM , TOP:Pd molar ratio= 2, and T (&#176;C) = 100. <a href="/files/ftp_upload/57667/57667fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57667/57667fig2.jpg" /> 
Figure 2. The schematic of ligand-mediated nucleation and growth model. In this proposed model, the capping ligands can associate and dissociate from both the metal precursor and nanoparticle surface, thereby, affecting the nucleation and growth kinetics (through altering the concentration of kinetically active precursor and the number of free surface sites, respectively). <a href="/files/ftp_upload/57667/57667fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57667/57667fig3.jpg" /> 
Figure 3. Experimental and ligand-based modeling results for the synthesis of Pd nanoparticles in toluene: (A) concentration of Pd atoms and (B) concentration of nanoparticles. The rate constants are summarized in Table 1. Experimental condition: [Pd(OAc)2]=25 mM , TOP:Pd molar ratio= 2, and T (&#176;C) = 100. <a href="/files/ftp_upload/57667/57667fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57667/57667fig4.jpg" /> 
Figure 4. (A) The rates of the nucleation and growth extracted from the ligand-based model for the synthesis of Pd nanoparticles in toluene and (B) <img alt="Equation 77" src="/files/ftp_upload/57667/57667eq77v4.jpg" /> ratio. Experimental condition: [Pd(OAc)2]=25 mM , TOP:Pd molar ratio= 2, and T (&#176;C) = 100. <a href="/files/ftp_upload/57667/57667fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57667/57667fig5.jpg" /> 
Figure 5. Experimental and ligand-based modeling results for the synthesis of Pd nanoparticles in pyridine: (A) concentration of Pd atoms and (B) concentration of nanoparticles. The rate constants are summarized in Table 1. Experimental condition: [Pd(OAc)2]=2.5 mM , TOP:Pd molar ratio= 2, and T (&#176;C) = 100. <a href="/files/ftp_upload/57667/57667fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57667/57667fig6v2.jpg" /> 
Figure 6. Model prediction of final nanoparticle size as a function of precursor concentration in pyridine solution (experimental data from Mozaffari et al.14). The error bars represent the standard deviation of the particle size distribution. Experimental condition: TOP:Pd molar ratio= 2, and T (&#176;C) = 100. <a href="/files/ftp_upload/57667/57667fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>k1-nuc</td>
			<td>k2-growth</td>
			<td>k3-f &#160;(A+L)</td>
			<td>k4-f &#160;(B+L)</td>
			<td>K5-eq&#160; (A+L)</td>
			<td>K6-eq&#160; (B+L)</td>
		</tr>
		<tr>
			<td>Units</td>
			<td>s-1&#160;&#160;</td>
			<td>L.mol-1.s-1</td>
			<td>L.mol-1.s-1</td>
			<td>L.mol-1.s-1</td>
			<td>L.mol-1</td>
			<td>L.mol-1</td>
		</tr>
		<tr>
			<td>25 mM Pd in Toluene</td>
			<td>1.8&#215;10-5</td>
			<td>10&#215;10-1</td>
			<td>4.7&#215;10-3</td>
			<td>3&#215;10-1</td>
			<td>1.5&#215;101</td>
			<td>1&#215;103</td>
		</tr>
		<tr>
			<td>2.5 mM Pd in Pyridine</td>
			<td>1.74&#215;10-5</td>
			<td>2.34&#215;101</td>
			<td>1.7&#215;10-1</td>
			<td>2.13&#215;10-2</td>
			<td>3.54&#215;102</td>
			<td>1.44&#215;102</td>
		</tr>
	</tbody>
</table>
Table 1. The extracted rate constants for Pd nanoparticle synthesis in different solvents (toluene and pyridine). Experimental condition: TOP:Pd molar ratio= 2, and T (&#176;C) = 100.

Watch this Scientific Journal Video about Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles at JoVE.com

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

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10.3K Views.  Virginia Polytechnic Institute and State University. The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

Video: Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Using Polystyrene-block-poly(acrylic acid)-coated Metal Nanoparticles as Monomers for Their Homo- and Co-polymerization

We present a template-free method for &#8220;polymerizing&#8221; nanoparticles into long chains without side branches. A variety of nanoparticles are encapsulated in polystyrene-block-poly(acrylic acid) (PSPAA) shells and then used as monomers for their self-assembly. Spherical PSPAA micelles upon acid treatment are known to assemble into cylindrical micelles. Exploiting this tendency, the core-shell nanoparticles are induced to aggregate, coalesce, and then transform into long chains. When more than one type of nanoparticles are used, random and block &#8220;copolymers&#8221; of nanoparticles can be obtained. Detailed procedures are reported for the PSPAA encapsulation of nanoparticles, homo- and co-polymerization of the core-shell nanoparticles, separation and purification of the resulting nanoparticle chains. Transformations of single-line chains into double- and triple-line chains are also presented. The synergy between the polymer shell and the embedded nanoparticles leads to an unusual chain-growth polymerization mode, giving long nanoparticle chains that are distinct from the products of the traditional step-growth aggregation process.

Using Polystyrene-block-polyacrylic acid-coated Metal Nanoparticles as Monomers for Their Homo- and Co-polymerization

We report protocols for &#8220;polymerizing&#8221; various types of polymer-encapsulated metal nanoparticles into long chains of &#8220;homo-&#8220; and &#8220;co-polymers&#8221;.

We present a template-free method for &#8220;polymerizing&#8221; nanoparticles into long chains without side branches. A variety of nanoparticles are ...

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 &#176;C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

A &#8220;removable ceramic coating method&#8221; is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The ...

Generation of Zerovalent Metal Core Nanoparticles Using n-(2-aminoethyl)-3-aminosilanetriol

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of n-(2-aminoethyl)-3-aminosilanetriol is presented. This compound can be used to effectively reduce and complex metal salts into metal core nanoparticles coated with the compound. By controlling the concentrations of salt and silane one is able to control reaction rates, particle size, and nanoparticle coating. The effects of these changes were characterized through transmission electron microscopy (TEM), UV-Vis spectrometry (UV-Vis), Nuclear Magnetic Resonance spectroscopy (NMR) and Fourier Transform Infrared spectroscopy (FTIR). A unique aspect to this reaction is that usually silanes hydrolyze and cross-link in water; however, in this system the silane is water-soluble and stable. It is known that silicon and amino moieties can form complexes with metal salts. The silicon is known to extend its coordination sphere to form penta- or hexa-coordinated species. Furthermore, the silanol group can undergo hydrolysis to form a Si-O-Si silica network, thereby transforming the metal nanoparticles into a functionalized nanocomposites.

Generation of Zerovalent Metal Core Nanoparticles Using n-2-aminoethyl-3-aminosilanetriol

A novel method for metal core nanoparticle synthesis using a water stable silanol is described.

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of ...

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. They offer new possibilities for the characterization of living systems and are particularly appropriate for detecting, localizing and quantifying the presence of metal oxide nanoparticles both in biological specimens and the environment. Indeed, these techniques all meet relevant requirements in terms of (i) sensitivity (from 1 up to 10 &#181;g.g-1 of dry mass), (ii) micrometer range spatial resolution, and (iii) multi-element detection. Given these characteristics, microbeam chemical element imaging can powerfully complement routine imaging techniques such as optical and fluorescence microscopy. This protocol describes how to perform a nuclear microprobe analysis on cultured cells (U2OS) exposed to titanium dioxide nanoparticles. Cells must grow on and be exposed directly in a specially designed sample holder used on the optical microscope and in the nuclear microprobe analysis stages. Plunge-freeze cryogenic fixation of the samples preserves both the cellular organization and the chemical element distribution. Simultaneous nuclear microprobe analysis (scanning transmission ion microscopy, Rutherford backscattering spectrometry and particle induced X-ray emission) performed on the sample provides information about the cellular density, the local distribution of the chemical elements, as well as the cellular content of nanoparticles. There is a growing need for such analytical tools within biology, especially in the emerging context of Nanotoxicology and Nanomedicine for which our comprehension of the interactions between nanoparticles and biological samples must be deepened. In particular, as nuclear microprobe analysis does not require nanoparticles to be labelled, nanoparticle abundances are quantifiable down to the individual cell level in a cell population, independently of their surface state.

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

We describe a procedure for the detection of chemical elements present in situ in human cells as well as their in vitro quantification. The method is well-suited to any cell type and is particularly useful for quantitative chemical analyses in single cells following in vitro metal oxide nanoparticles exposure.

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. ...

High Resolution Physical Characterization of Single Metallic Nanoparticles

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single nanometer-scale pore. The signal is characteristic of the molecule&#39;s physicochemical properties and its interactions with the pore. We demonstrate that the nanopore formed by the bacterial protein exotoxin Staphylococcus aureus alpha hemolysin (&#945;HL) can detect polyoxometalates (POMs, anionic metal oxygen clusters), at the single molecule limit. Moreover, multiple degradation products of 12-phosphotungstic acid POM (PTA, H3PW12O40) in solution are simultaneously measured. The single molecule sensitivity of the nanopore method allows for POMs to be characterized at significantly lower concentrations than required for nuclear magnetic resonance (NMR) spectroscopy. This technique could serve as a new tool for chemists to study the molecular properties of polyoxometalates or other metallic clusters, to better understand POM synthetic processes, and possibly improve their yield. Hypothetically, the location of a given atom, or the rotation of a fragment in the molecule, and the metal oxidation state could be investigated with this method. In addition, this new technique has the advantage of allowing the real-time monitoring of molecules in solution.&#160;

Here, we present a protocol to detect discrete metal oxygen clusters, polyoxometalates (POMs), at the single molecule limit using a biological nanopore-based electronic platform. The method provides a complementary approach to traditional analytical chemistry tools used in the study of these molecules.

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single ...

Molten-Salt Synthesis of Complex Metal Oxide Nanoparticles

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. Here, we introduce the molten-salt synthesis (MSS) method for making metal oxide nanomaterials. Advantages over other methods include its simplicity, greenness, reliability, scalability, and generalizability. Using pyrochlore lanthanum hafnium oxide (La2Hf2O7) as a representative, we describe the MSS protocol for the successful synthesis of complex metal oxide nanoparticles (NPs). Furthermore, this method has the unique ability to produce NPs with different material features by changing various synthesis parameters such as pH, temperature, duration, and post-annealing. By fine-tuning these parameters, we are able to synthesize highly uniform, non-agglomerated, and highly crystalline NPs. As a specific example, we vary the particle size of the La2Hf2O7&#160;NPs by changing the concentration of the ammonium hydroxide solution used in the MSS process, which allows us to further explore the effect of particle size on various properties. It is expected that the MSS method will become a more popular synthesis method for nanomaterials and more widely employed in the nanoscience and nanotechnology community in the upcoming years.

Here, we demonstrate a unique, relatively low-temperature, molten-salt synthesis method for preparing uniform complex metal oxide lanthanum hafnate nanoparticles.

The development of feasible synthesis methods is critical for the successful exploration of novel properties and potential applications of nanomaterials. ...

Synthesis and Characterization of Amphiphilic Gold Nanoparticles

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of their interactions with cell membranes, lipid bilayers, and viruses. The hydrophilic ligands make these particles colloidally stable in aqueous solutions and the combination with hydrophobic ligands creates an amphiphilic particle that can be loaded with hydrophobic drugs, fuse with the lipid membranes, and resist nonspecific protein adsorption. Many of these properties depend on nanoparticle size and the composition of the ligand shell. It is, therefore, crucial to have a reproducible synthetic method and reliable characterization techniques that allow the determination of nanoparticle properties and the ligand shell composition. Here, a one-phase chemical reduction, followed by a thorough purification to synthesize these nanoparticles with diameters below 5 nm, is presented. The ratio between the two ligands on the surface of the nanoparticle can be tuned through their stoichiometric ratio used during synthesis. We demonstrate how various routine techniques, such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and ultraviolet-visible (UV-Vis) spectrometry, are combined to comprehensively characterize the physicochemical parameters of the nanoparticles.

Amphiphilic gold nanoparticles can be used in many biological applications. A protocol to synthesize gold nanoparticles coated by a binary mixture of ligands and a detailed characterization of these particles is presented.

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of ...

Synthesis Method for Cellulose Nanofiber Biotemplated Palladium Composite Aerogels

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often result in fragile aerogels with poor shape control. The use of carboxymethylated cellulose nanofibers (CNFs) to form a covalently bonded hydrogel allows for the reduction of metal ions such as palladium on the CNFs with control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Crosslinking the carboxymethylated cellulose nanofibers is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine. The CNF hydrogels maintain their shape throughout synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO2 supercritical drying. Varying the precursor palladium ion concentration allows for control over the metal content in the final aerogel composite through a direct ion chemical reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in other sol-gel techniques. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metallized palladium porous structure.

A synthesis method for cellulose nanofiber biotemplated palladium composite aerogels is presented.The resulting composite aerogel materials offer potential for catalysis, sensing, and hydrogen gas storage applications.

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often ...